This invention is a high voltage field effect transistor (HIFET), and the process for making said HIFET. Its radial configuration occupies little space on a substrate and minimizes cross conduction between devices, and is, therefore, suitable for inclusion in LSI designs requiring high voltage outputs.
It is frequently required, in industrial applications of large scale integrated circuits, to switch high voltages. Since LSI devices commonly work with supply voltages in the area of from four to fifteen volts, special circuits must be provided to handle voltages in the 200 to 1,000 volt range. An example of this use would be an electrostatic printer. In this case, large amounts of low voltage random circuitry are required to decode signals arriving from an outside source, and a voltage of at least 200 volts must be applied to the electrostatic printing mechanism.
The common method of fabricating this kind of circuit would be to use an LSI chip for the low voltage logic functions and to supply a separate high voltage amplifier for each output channel. To use a common numerical example, if there were sixteen output lines to be driven, the LSI chip output would be connected to sixteen discrete voltage amplifiers, each of which would drive an output channel.
If these high voltage amplifiers could be included within the LSI chip a large reduction in cost could be achieved, because the one consolidated unit would be far less costly than the original seventeen units, and because of the reduced amount of space and wiring required. Also, the reliability of the resultant circuit would be far greater than the reliability of the original circuit.
However, the inclusion of these high voltage amplifiers on an LSI chip has been impractical because of the large substrate area that each amplifier requires and because of the inherent tendency of high voltage devices to interact with each other and with other circuits on the same chip. To produce a commercial chip which includes a large number of high voltage output circuits would therefore require a circuit configuration which would allow the dense packing of high voltage amplifiers into an LSI chip but which would not allow these amplifiers to interact with each other or with the remainder of the circuitry on the chip.